Device for adjusting the refractive power of electron lenses operating with permanent magnet excitation



E. RUSKA 2,804,548 DEVICE FOR ADJUSTING was REFRACTIVE POWER OF ELECTRON Aug. 27, 1957 LENSES OPERATING WITH PERMANENT MAGNET EXCITATION Filed Jan. 19, 1952 3 Sheets-Sheet 1 I Inn enter? zfwjzm.

Aug. 27, 1957 us 2,804,548

DEVICE FOR ADJUSTING THE REFRACTIVE POWER OF ELECTRON LENSES OPERATING WITH PERMANENT MAGNET EXCITATION Filed Jan. 19, 1952 5 Sheets-Sheet 2 Inventor: J26; filwtw.

2,804,548 WER OF ELECTRON 3 Sheets-Sheet Z Aug. 27, 1957 DEVICE FOR ADJUSTING THE REFRACTIVE P0 LENSES OPERATING WITH PERMANENT MAGNET EXCITATION Filed Jan. 19, 1952 9 6 5 467 1 4 v L m,N\\\\\\\\. M 5\\\\\\\\\NA\\ 4 E 3% x 2 2 2 Eli-N w l V\ 5 ----N n. a N\\\\\\\\\\\\ .s //JA/ Inventor: 4 523%,,

United States DEVICE F OR ADJUSTENfi THE REFRACTHVE POWER OF ELECTRUN LENSES OPERAT- ING WITH PERMANENT MAGNET EXCH- TATION Public Law 619, August 23, 1954 Patent expires October 1, 1968 This invention is concerned with a device for adjusting the refractive power of electron lenses operating with permanent magnet excitation, and particularly electron lenses of this kind as they are used in electron microscopes.

Electron microscopes with high resolving power have been made in recent years, making it possible to produce from a very small specimen a highly magnified image of an exceedingly minute range of such specimen. However, it is not always desirable, in working with such instruments, to utilize the highest magnification; there is need for structures which permit alteration or adjustment of the final image magnification.

Various widely different suggestions have been made for solving this problem. For example, in electron microscopes operating with electromagnetically excited lenses, an alteration of the final image magnification was obtained by continuous regulation of the exciting current for the projective. The regulation was, however, not fully satisfactory so far as the quality of the images was concerned, and it was subsequently suggested to associate with the projective a plurality of mutually exchangeable pole shoe sets in which the pole shoe spacing, and in some structures also the pole shoe bores, were of different sizes, thus making it possible to selectively adjust in rough stages, for example, three different final image magnifications. Another structure provided a special electromagnetic regulation lens between the objective with high resolving power and the projective, so as to realize the desirability of always fully utilizing the final image size. The final image magnification is in such structures varied only by adjustment of this intermediate regulation lens, and the operator is therefore enabled to fully illuminate upon the viewing screen a certain size, that is to say, to full utilize the final picture size of the associated photographic plate.

No satisfactory solution has heretofore been found for the problem in the case of lens structures with permanent magnet excitation.

The invention proposes a particularly advantageous solution for the above indicated problem by the provision of an electronic lens operating with permanent magnet excitation, which is arranged for continuous regulation of the refractive power. In accordance with the inven tion, the regulation of the refractive power of the lens is obtained by a relative motion in the direction of the longitudinal axis of the permanent magnet, between the magnet and such of the soft iron parts through which the flux extends from the magnet to the lens gap. The use of this principle makes it possible to carry out a regulation of the refractive power of the lens to a considerably greater extent than in structures made in accordance with previous proposals which suggested to provide in the magnetic circuit a variable magnetic resistance. This is of particular importance in using the regulation of the refractive power for the regulation of the final image mapification.

atent' 2,854,548 Patented Aug. 27, 1957 In accordance with another object of the invention, the magnetostatic lenses are preferably formed so that a bridge carrying the magnetic flux between the permanent magnet and the parts belonging to the magnetic circuit serves as a regulating body, said bridge being movable in the direction of the longitudinal axis of the stationary permanent magnet in such a manner that a variable part of the total flux becomes efiective at the lens gap.

An electronic lens which is regulable in accordance with the invention is arranged in the optics of an electron microscope in which three lens gaps are serially disposed in the beam path in such a manner that the permanent magnets serving for the excitation of the objective and of the projective and a regulating body associated with the intermediate regulation lens, are so dis posed that a continuous regulable flux is conducted from the permanent magnets of the objective and the projec tive to the intermediate regulation lens by adjustment of the regulating body, while the magnetic flux at the objective and the projective are left unaltered.

In case pole shoe lenses are used in realizing the invention, the pole shoes between which there is no lens gap are magnetically connected, and the two outer lenses (the objective and the projective) are each associated with at least one permanent magnet system, for example, a permanent magnet rod. These systems are for the regulation of the centrally disposed lens coupled by the regulating body which forms the bridge between the permanent magnet systems. The device is preferably so constructed that the regulation can be carried out within the limits between zero and the sum of the flux of the permanent magnet systems.

The arrangement produces a permanent magnet system for a three-stage pole shoe lens structure, permitting a continuous regulation of the centrally disposed lens, preferably within the above mentioned wide limits, Without affecting the refractive power of the two outer lenses. Permanent magnet rods disposed parallel to the beam path may be used for exciting the two permanent magnet systems. These magnet rods are preferably longer than the pair of pole shoes associated with the outer lenses. The arrangement may be advantageously so constructed that the flux from one pole of one magnet rod system is conducted through bent magnetic yokes, in each case to a pole shoe member carrying pole shoes of adjacent lenses, and that a partial flux from this pole shoe member reaches the other pole of the magnetic rod through the outer lens and a yoke piece extending in a rectilinear plane, while a parallel partial flux flows over the centrally disposed lens.

A compact embodiment, which is particularly suitable for assembly with the remaining parts of the apparatus, will result from the use of dishor disk-shaped yokes for the pole shoes at the opposite ends of the lens system. The marginal portions of these dish-shaped yokes can then be connected with one pole of the permanent magnet rod or rods. Two bent yoke members may be employed, one associated with each of the pole shoe members which carry pole shoes of two adjacent lenses, these bent yoke members connecting the corresponding pole shoes each with two diagonally oppositely disposed identical poles of two magnetic rods, the two bent yoke members being disposed one relative to the other in the manner of two adjacent links of a chain.

The invention may also be realized by using for the excitation of the three lens gaps only one or two rodshaped permanent magnets. In the above mentioned structure with the dish-shaped yoke members it may be advantageous, for reasons of symmetry, to use four permanent magnet rods in pairs, the rods of each pair 70. diagonally oppositely disposed. One pair of these rods may be positioned with the north pole upon one dishshaped yoke member and the other pair with the south pole upon the other dish-shaped yoke member. The remaining poles of the rods are connected with the bent yoke members. The previously indicated flux-regulating bridge, which is important for the invention, is in this latter case. suitably made of two parts in such a manner that the two rodlike permanent magnet pairs coact in the regulation in identical manner.

Other embodiments contemplate, for the excitation of the two permanent magnet systems, permanent magnet rods which are parallel to one another and transverse to the axis of the beam path, theflux-regulating bridge being connected therewith.

The invention is not limitedto the use of. rectilinear permanent magnet rods. Arcuaterods disposed transverse of the axis of the beam pathmay beused. While the preferred. embodiment-employs.fixedly disposed permanent magnet rods and a flux-regulating. bridge, which is arranged for continuous adjustment relative thereto, it is understood that the inventive thoughts may likewise be realized by making the permanent magnet rods movable relative to the flux-regulating. bridge. Whetherthe bridge or the permanent magnet-rodsare movably disposed, the regulation is preferably always so that-the bridge in one'of its terminal positions provides a mag netic flux for the two poles of the-magnetic system, which are associated with the central lens, and that this bridge is incident to the upwardv regulation displaced relative to the two permanent magnet systems until it is in alternate terminal position in which it bridges the two other poles which are associated with the outer pole shoes of the two outer lenses.

The invention and its various objects and featureswill become apparent from the description of embodiments, which will presently be rendered with reference to the accompanying drawings. In these drawings,

Fig. 1 shows in schematic perspective partly sectional view an embodiment of the invention as applied to an image-forming lens system having three lenses in the beam path, employing permanent magnet rods which extend in parallel with the axis of the beampath;

Fig. la is a transverse sectional view through the easing (omittedin Fig. 1) showing the parts of Fig. 1 at a level looking down at certain bridge members and showing some of these parts in elevation and others in section. For the sake of convenience, Fig. la hasbeen shown angularly displaced by about 45 relative to Fig. 1',

Fig; 2. illustrates an embodiment employing permanent magnet rods which extend parallel to one another, but

transverse to the beam axis;

Fig. 3 indicates-in similar schematic manner an embodiment employing for the regulation two arcuate permanent magnet rods which are disposed in a plane trans- -verse to the axis of the beam;

Fig. 4 shows a furtherembodiment in which the fiuxregulating bridge member is fixedly disposed, while twoc oacting permanent magnets are arranged relatively movable thereto;

Fig; 5 illustrates in schematic manner an embodiment employing a rod-shaped permanent magnet having an axially directedextension, the magnet with its extension being movable for accomplishing the flux-regulating 0peration; and

Fig. 6 is a diagrammatic sectional view of an electron microscope employing regulating means in accordance with the structure shown in Fig. 2.

The form of the invention shown in Figs. 1 and 1a is applied to a system which is excited with permanent magnets having pole shoes of generally known structure; The upper lens (objective) is provided with a dish-shaped pole shoe member 1. Numeral 2 designates a ring of nonmagnetic material, which is associated with the gap of the lens having as one pole shoe the pole shoe member 1. The next successive pole shoe member forms the lower pole'shoe'3 of the outer (objective) lens, and atits lower end the upper pole shoe of the central or intermediate lens. The latter is associated with the nonmagnetic ring 4 and the pole shoe 5 which is formed at the upper end of a pole shoe member, also forming at its lower end the upper pole shoe for the next successive lens (projective). A nonmagnetic ring 6 is associated with the lens gap of this next successive lower lens. The pole shoe member 7, forming the lower pole shoe of the lower lens, is again a dish-shaped member similar to the member 1 of the upper lens.

For the excitation of this magnet system there are provided four permanent magnet rods which are disposed in pairs, the rods of each pair being positioned diagonally opposite. The rods 8 and. 9 with their ends forming the north poles are fastened on the dish-shaped upper pole shoe member 1, while the rods 10*and 11 are fastened with their south poles in engagement with the lower dishshaped pole shoe member 7. A bent yoke 12 extends from the south poles of the rods 8 and 9, the central portion of this yokeconnecting'with the pole'shoe 3 so'that the magnetic flux flows from the north poles of the rods 8 and 9 through the upper dish-shaped pole shoe member- 1, then over the gap of the upper'or outerlens and'back to the south poles through the yoke 12. Yoke 13' is similarly bent in generally U- or V-shaped fashion'to form a central portion which connectswith' the pole shoe member 5, its legs connecting withth'e two north pole ends of the magnet rods 10 and 11. The two yokes 12 and 13 are disposed relative to one another in the manner of twoadjaeent links of a chain. This structure and arrangement permits-reduction of stray magnetic eifects to a very small value. The magnetic rods 10 and 11 thus serve for the excitation or energization of the lower lens.

The dimensions of the magnetic rods and of the yokes are such that the free ends of the yokes do not touch the dish-shaped yoke members 1 and'7 at the opposite outer ends of the structure.

The two permanent magnet rods 9 and 10 are coupled" by a flux-regulating bridge member 15 which is rotatable in the direction of the arrowshown in Fig. 1. A similar flux-regulating bridge member 16 is provided for coupling the rods 8 and 11.- The two flux-regulating bridge members 15 and 16 are by suitable gear means coupled with each other, so that they may be displaced in identical direction of regulation. This gear means may include any known and suitable gear and pinion means or the like, as well as any suitable and known drive means therefor, which permits the operator to carry out angular adjustment of the two flux-regulating bridge members. In order to obtain a good transition of the flux from the magnet rods to the two flux-regulating bridge'members, the rods are provided with fiat surfaces, as shown in the drawing, these flat surfaces facing the coacting fluxregulating bridge members.

An example of the gear means "that may be used for actuating the flux-regulating bridge members 15 and 16 is apparent from Fig. la. The soft iron bridge member 15 carries a pinion 70 keyed to the shaft 72 which is journalled'in the casing wall (omitted in Fig.1). With the pinion 70 meshes a pinion 71 on the shaft 75 which is journalled in the casing wall and carriesoutside thereof a pinion 77. The latter meshes with the pinion 76 carried on the shaft 74 which is journalled in bearing'members extending from the casing Wall as shown and'provided with a knob 73. Accordingly, rotation of the knob 73 will rotate thebridge member 15 relative to the magnet.

rods'9 and 10, through the. medium of' the pinions 76, 77, 71, 70. The rotation of the soft iron bridge member 16 relative to the magnet rods 11 and 8' is accomplishedsimilarly' through the'mediumiof the pinions 78, 79, 81 and 82.

In the position shown in Figs. 1 and 1a; the bridge members 15 and 16' form a magnetic pathextending rela-- tive to the air gap of the central lens, so that the refractive power of this central lens is zero; When the bridge members are rotated in the direction of the arrow 14 shown in Fig. 1, the flux on the central lens gap is continuously or progressively increased until it corresponds to the sum of the flux of both permanent magnet systems, when the bridge members assume a position approximately ninety degrees from the positions shown in the drawing. In this latter terminal position the two dishshaped pole shoe members 1 and 7 are bridged by the opposite ends 17 and 18 of the flux-regulating bridge members, and the flux at the central lens gap therefore corresponds to the product of the flux of the rods 19 and 11 and that of the rods 8 and 9. The stray magnetic effect is kept as small as possible by the form and manner of arrangement of the yokes 12 and 13.

In the structure shown in Fig. 2, the permanent magnet rods for the excitation of the permanent magnet system extend in parallel to one another, but transverse to the axis of the beam path. Numerals 21, 22, 23 designate the three lens gaps which are disposed serially in the beam path. The pole shoe member 24 operates in two directions; that is, it forms the lower pole shoe of the objective lens 21 and the upper pole shoe of the intermediate lens 22. It is connected with the south pole of the rod-shaped permanent magnet 25. The pole shoe member 26, which forms the lower pole shoe of the intermediate lens 22 and the upper pole shoe of the projective lens 23, is connected with the north pole of a similar permanent magnet rod 27. From the rod 25 extend the yoke parts 28 and 29 to the upper pole shoe 30 of the lens 21; and from the south pole of the rod 27 extend the yoke parts 31 and 32 to the lower pole shoe 33 of the lens 23. The magnet rods 25 and 27 are thus associated with the upper lens 21 and the lower lens 23, respectively. The two magnet systems formed by the rods 25 and 27 are for the excitation of the central or intermediate lens 22 coupled over the flux-regulating bridge member 34. This member is made of soft iron and forms a flux bridge between the parts of the magnet rods with which it is at any time in engagement. Suitable means (shown in Fig. 6) is provided for moving the flux-regulating member 34 in the direction of the double arrow. In its dotted terminal position 35, the member 34 forms a bridge between the south pole of the rod 25 and the north pole of the rod 27, thus shunting or short-circuiting the intermediate lens 22 so that the refractive power of this lens is zero. If the bridge member 34 is moved to the right, the refractive power of the intermediate or central lens will gradually increase from zero to a maximum value when the bridge member 34 reaches the terminal position in which it is shown in dotted lines designated by the numeral 36. In this terminal position, the north pole of the rod 25 is directly connected with the south pole to the rod 27, so that there will be at the lens gap 22 a magnetic flux which is a product of the magnetic flux of both permanent magnet rods. The means for displacing the flux-regulating bridge member 34 may again be any suitable known gear and guide means as well as drive means for obtaining the operation as desired. An example of gear means that may be used will be explained in connection with Fig. 6.

In the embodiment according to Fig. 3, there are provided two arcuate permanent magnet rods 41 and 42. Parts shown in this figure, which correspond to the form of invention as shown in Fig. 1, are indicated by like reference numerals. The south pole end of the magnet 41 is connected to the dish-shaped member 1 by means of the holder member 43, and its north pole end connects with the pole shoe 3 over the holder member 45. The south and north pole ends of the magnet 42 are in similar manner connected with the pole shoe member 5 and with the dish-shaped member 7 by means of the holder members 46 and 44, respectively. The magnetic flux is conducted over these holders 4346, inclusive. A soft iron bridge or flux-regulating member 47 is provided for the coupling of the two magnetic systems. This member is for the regulation of the refractive power of the central or intermediate lens adjustable along a circular path concentric to the axis of the beam. As in the former embodiments, any suitable and known gear, drive and guide means may be provided for carrying out the displacement and adjustment of the flux-regulating member 47.

The structure shown in Fig. 4 comprises the flux-regulating bridge member 62 which is suitably fixedly disposed and two rodlike magnets 59 and 61 which are by known and suitable means disposed movable or adjustable relative thereto. There are, as in the former embodiments, three lens gaps 51, 52, 53 which are seriallydisposed in the beam path. The input is formed by the dish-shaped pole shoe member 54 and the output by the similarly dish-shaped pole shoe member 57. Numerals 55 and 56 are the two pole shoe members which are operative in two directions; that is, each of these pole shoe members forms at the opposite ends pole shoes for the associated lenses. The magnetic rods 59 and 61 are mounted on journals 58 and 60, respectively, these journals embracing the pole shoe members 55 and 56, as shown. The spirally formed arcuate flux bridge member 62 is fixedly disposed between the two rotatable magnets 59 and 61. These magnets touch with their upper and lower edges the corresponding marginal edges of the dishshaped end members 54 and 57 and of the flux bridge member 62, respectively. The two magnets 59 and 61 are individually movable, and there is therefore in this embodiment a particularly great adjustment range available.

The nonmagnetic rings associated with the lens gaps have been omitted from Fig. 4 in order to show the pole shoes. The means for rotating the magnet rods 59 and 61 may again include any desired and suitable gear means. For example, the journals 58 and 60 may be provided with gear teeth or the like meshing with a coacting gear so as to bring about rotation thereof for the purpose described, and the coacting gear may be associated with suitable drive means which is operable from the outside.

The pole shoes, together with the nonmagnetic spacing rings, form in all embodiments the vacuum walls in the range of the lens system of the corresponding structures. The construction may, however, be such that the vacuum wall is formed by alternate tubular magnetic and nonmagnetic parts which carry the pole shoes and pole shoe systems, respectively. In order to obtain good evacuation, one or more tubular pump ducts may be provided in known manner.

While the structures so far described employ soft iron parts belonging to the magnetic circuit, which are adjustable relative to stationary parts of the corresponding lens systems, it is possible to provide, as indicated in Fig. 5, a rod-shaped permanent magnet (from which the magnetic flux is conducted to the lens gap) alongside of one pole, and a soft iron extension projecting therefrom, the length of which corresponds to the length of its body, with means for axially moving the entire rod with its extension so as to carry out the desired regulation. The dish-shaped end members are partially shown in Fig. 5 in section and designated by numerals 121 and 122. Between these end members are the lenses, as already described with reference to Fig. 1, these lenses having been omitted from Fig. 5 to keep it simple. A permanent magnet rod is provided for exciting the system. Projecting from the north pole end of this rod is a soft iron extension rod 126, the length of which corresponds to the rod 125. This rod system 125-126 is movably mounted in the dish-shaped members, as indicated at 123124. Suitable gear means, as mentioned before, are provided for axially displacing the rods 125126 as a unit to vary the magnetic flux of the lens arrangement for the purpose already described.

An example of an electron microscope employing the regulation means explained with reference to Fig. 2 is diagrammatically shown in Fig. 6. Numeral 201 indicates the cathode, 220 is the Wehnelt cylinder, 203 the anode, and 204 indicates the specimen holder containing the specimen 205'. The soft iron bridge members 34-are provided with threaded bores engaged by the screws 206 which are suitably journalled in the casing wall, each carrying a knob 207. The bridge members 34 are displaced responsive to rotation of the respective knobs. Numeral 208 is the duct extending to the'vacuum pump. 209'is the viewing screen, 210 indicates a photographic plate, and 211 is a sight opening. The remaining numerals in Fig. 6 indicate like numbered parts discussed in connection with Fig. 2.

Changes may be made within the scope and spirit of the appended claims.

Iclaim:

1. In an electron lens system having permanent magnet means for the excitation thereof, a device for the continuous regulation of the refractive power of a lens of said lens system, said device comprising a soft iron member in-contact with said permanent magnet means, the magnetic flux flowing from said permanent magnet means through said soft iron member to the lens gap of said lens, and means for carrying out relative displacement between said permanent magnet means and said soft iron member to displace the point at which the magnetic flux passes from said permanent magnet means to said soft iron member in the direction of the inner field lines of said permanent magnet means.

2. The structure as set forth in claim 1, wherein said permanent magnet means is fixedly mounted, said soft iron member forming a bridge for the magnetic flux from said permanent magnet means to said lens gap, and means for disposing said soft iron member for displacement in the direction of the inner field lines of said fixedly mounted permanent magnet means for the purpose of making effective in the magnetic circuit comprising said lens gap a variable part of the total magnetic flux of said permanent magnet means. 7

3. The structure as defined in claim 1, wherein said permanent magnet means comprises a plurality of rodlike permanent magnets, and means for disposing said rodlike permanent magnets movable relative to said soft iron member.

4. The structure defined in claim 1, wherein said permanent magnet means comprises a rodlike permanent magnet disposed alongside one pole of said lens system, a rodlike extension projecting axially from said rodlike permanent magnet constituting said soft iron member, and means for mounting said permanent magnet with said extension for displacement in the direction of the longitudinal axis thereof.

5. The structure defined in claim 1, together with nonmagnetic rings associated with the pole shoes forming the lens gaps of said lens system and forming therewith the vacuum walls within the range of said lens system.

6. An electron microscope having a lens system comprising pole shoe members forming three lens gaps disposed serially in the path of the electron beam, said lens gaps being associated with the objective and with an intermediate lens and with the projective, respectively, permanent magnet means for exciting the objective and the projective, respectively, and a movable flux-regulating body associated with said intermediate lens and coacting with said permanent magnet means, said fluxregulating body forming a bridge for conducting to said intermediate lens magnetic flux from said permanent magnet means without altering the magnetic fiux at the objective and at the projective.

7. The structure as set forth in claim 6, together with means for magnetically connecting predetermined pole shoe members of said lens system, said flux regulating body forming a bridge between said permanent magnet means of said objective and said projective.

8. The structure as set forth in claim 6, together with means for magnetically connecting predetermined pole shoe members of said lens system, said flux-regulating body forming a bridge between the permanent magnet means of said objective and said-projective, said bridging in one of its terminal positions the poles of the permanent magnet means associated with said intermediate lens while bridging in its other terminal position the poles which are associated with the outer pole shoe members of the objective and projective, respectively.

9. The structure as set forth in claim 6, wherein rodlike permanent magnets disposed in parallel with the beam axis constitutes said permanent magnet means.

10. The structure as set forth in claim 6, wherein the length of said permanent magnet means exceeds the length of either of the pair of pole shoes associated with said objective and projective,- respectively.

II. The structure as set forth in claim 6, together with a bent yoke member, a yoke member extending in a rectilinear plane, the magnetic flux path extending from one pole of said permanent magnet means through said bent yoke member to a pole shoe member forming part of two adjacent lens gaps andthence through the outer lens of said lens gap and said yoke member which extends in the rectilinear plane back to the other pole of said permanent magnet means,- a partial parallel flux path extending over said intermediate lens.

12. The structure defined in claim 6, wherein generally dish-shaped yoke members form the outer pole shoes of the objective andthe projective, respectively, said permanent magnet means comprising a rodlike member which is connected with a marginal area of said dishshaped yoke members.

13. The structure defined in claim 6, together with two bent yoke members, one for each of two pole piece members of said lens system which carries pole pieces associated with two adjacent lenses, for connecting the associated pole piece member with identical poles of said permanent magnet means, said bent yoke members being disposed relative to one another in the manner of adjacent links of a chain.

14. The structure defined in claim 6, wherein four rodlike permanent magnet members form said permanent magnet means, said rodlike members being disposed in pairs in diagonal opposite relationship, dishlike yoke members, one for the objective and one for the projective, one pair of said rodlike permanent magnet members being connected with the north poles thereof in engagement with one of said dishlike yoke members and the other pair of rodlike members being connected with the south poles thereof in engagement with the other dishlike member, and a bent yoke member for interconnecting the remaining free ends of each respective pair of rodlike permanent magnet members.

15. The structure defined in claim 6, wherein said permanent magnet means comprises rodlike permanent magnet members disposed in parallel to one another and extending transverse to the axis of the beam, said rodlike permanent magnet members being connected with said flux-regulating body and the latter being disposed movable in the direction of the longitudinal axes of said rodlike permanent magnet members.

16. The structure defined in claim 6, wherein said permanent magnet means comprises arcuately shaped rodlike permanent magnet members.

17. The structure defined in claim 6, wherein said permanent magnet means comprises rotatable rodlike permanent magnet members, a fixedly disposed spirally extending member which constitutes said bridge being disposed between said rotatable rodlike permanent magnet members.

18. The structure defined in claim 6, together with alternate magnetic and nonmagnetic tubular members forming the pole shoes of the lens system and the vacuum wall therefor, respectively.

19. In an electron microscope, terminal pole shoe members forming the outer pole shoes of the objective and the projective, respectively, a pair of inner pole shoe members each forming a pair of pole shoes of successive lens gaps and of the lens gap of an intermediate lens, rodlike permanent magnet means connected with certain predetermined ones of said pole shoes for creating flux for the excitation of the lenses formed by said pole shoes, and a member coacting with said permanent magnet means for regulating the flux in said intermediate lens to regulate the refractive power thereof.

20. The structure defined in claim 19, wherein said last named member is movable in a plane which parallels the axis of the electron beam traversing said lenses.

21. The structure defined in claim 19, wherein said last named member is movable in a plane which extends transverse to the axis of the electron beam traversing said lenses.

i0 22. The structure defined in claim 19, wherein said regulating body is movable through an arcuate path relative to the axis of the electron beam traversing said lenses.

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